Coal is formed over time from decomposed and compressed organic matter. Coal is typically found in substantially horizontal seams extending between sedimentary rock strata such as limestone, sandstone or shale. Depending upon the position of the coal relative to the surface topography, surface and underground mining techniques are primarily utilized to recover this coal.
Surface or strip mining involves the removal of material, known as overburden, overlying a coal seam so as to expose the coal for recovery. In recent years, surface mining has gained prominence over underground mining in the United States. This is due to many factors including: (a) the increased material moving capacity of surface or strip mining equipment; (b) lower costs for surface mining than underground mining; (c) the better safety record of surface mining versus underground mining; (d) a higher coal recovery percentage for surface mining versus underground mining and (e) many coal reserves favor extraction by surface mining due to geological factors.
Despite these advantages, applications of surface mining are limited. The primary limiting factor is the depth of overburden. More particularly, the greater the depth the coal seam is below the surface, the greater the amount of overburden that must be removed to reach the coal. Eventually, the depth becomes so great that strip mining is simply economically unfeasible.
Where this occurs, large quantities of coal may still remain in the ground. A need therefore exists for mining methods that may be utilized for the economic recovery of this coal. Underground mining applications in such an instance are very limited. A number of factors are responsible for this including the possible existence of poor roof support conditions, the thinness of the seam and/or the presence of insufficient quantities of coal to warrant the large capital investments characteristic of underground operations.
As a result, auger mining has often been utilized in the past to recover coal following a strip mining operation. Advantageously, auger mining requires a relatively low capital expenditure when compared to either surface or underground mining. It also may be activated quickly and is very efficient providing more tons per man day than any other form of mining.
In auger mining, an auger is used to bore into the face of the seam and recover coal from beneath the overburden. It may be utilized in relatively thin seams and is safer than both surface or underground mining while also providing a generally cleaner coal product.
Despite all these advantages, however, auger mining suffers from a number of drawbacks that have left the coal industry looking for still another alternative to use in recovering coal left in the ground following strip mining. More particularly, despite the high rate of production, auger mining provides a relatively low total coal recovery. Lost coal recovery takes two main forms. In one instance, pillars of coal must be left standing between adjacent auger holes in order to support the overburden. In another instance, the penetration depth of augers is limited as augering equipment characteristically drills holes that sag downwardly gradually with increasing depth of penetration into the coal seam. In fact, holes drilled with conventional auguering equipment are usually only of a depth of 150 feet with 200 feet being rarely obtainable. Thus, any coal beyond this distance is not recoverable with auger equipment. As such, coal recovery for a resource area being augered is usually less than about 35%.
A need therefore exists for an improved mining apparatus that may be utilized to efficiently recover coal following a strip mining operation where the overburden becomes too costly to remove. Preferably, the apparatus is also effectively adapted for recovering coal from relatively thin seams. In addition, the apparatus could be utilized to increase coal recovery in underground mining operations by safely recovering the coal in pillars normally left behind in a room and pillar mining operation. In this way, the apparatus may be effectively utilized to increase the total coal recovery under any number of circumstances and conditions in a relatively cost efficient manner.